1. Field of the Invention
The present invention relates to a technique to correct a flicker component in an image captured using an image sensor.
2. Description of the Related Art
In an imaging apparatus employing an image sensor, periodic light and dark changes may appear in a captured image when an object under fluorescent lighting is continuously photographed for a plurality of frames (or fields) as a moving image or continuous still images. This phenomenon is referred to as a fluorescent flicker, which occurs due to a difference between the frequency of the commercial power supply and a vertical synchronizing frequency of the imaging apparatus.
Conventionally, a technique has been proposed which detects a periodic fluctuation component of a signal level (hereafter referred to as a flicker component) and corrects a flicker by applying the inverse gain of the detected flicker component to the image signal (refer to Japanese Patent Application Laid-Open No. 11-122513).
The flicker component of a fluorescent flicker in an imaging apparatus varies depending on the frequency of the power supply and the light emission properties. Furthermore, a driving state of an imaging apparatus, such as accumulation time and a frame rate of an image sensor and a change in reading time due to an adding process and a thinning process of pixels, also causes variation in the flicker component. The driving state of an imaging apparatus is changed or controlled by an imaging system or in response to instructions from a user. Therefore, the driving state can rapidly change. In this case, the flicker component can significantly change with a change in the driving state.
However, a correction method discussed in Japanese Patent Application Laid-Open No. 11-122513 is a method of detecting a flicker component using an intensity distribution in the vertical direction generated by integrating an image signal of the current and past frames (or fields) in the horizontal direction. In such a method, it is difficult to follow an abrupt change in flicker component and to appropriately correct a flicker, because a time difference occurs between an image signal used to detect the flicker and an image signal to be corrected. Thus, it is difficult to execute suitable correction for a flicker. On the contrary, the correction can sometimes adversely affect a captured image.